1. Field of the Invention
This invention relates generally to a semiconductor technique and more particularly to a silicone polymer film used as a low-k (low dielectric constant) hard film on a semiconductor substrate, which is formed by using a plasma CVD (chemical vapor deposition) apparatus.
2. Description of the Related Art
Because of the recent rise in requirements for the large-scale integration of semiconductor devices, a multi-layered wiring technique attracts a great deal of attention. In these multi-layered structures, however, capacitance among individual wires hinders high speed operations. In order to reduce the capacitance it is necessary to reduce relative dielectric constant of the insulation film. Thus, various materials having a relatively low relative dielectric constant have been developed for insulation films.
Conventional silicon oxide films SiOx are produced by a method in which oxygen O2 or nitrogen oxide N2O is added as an oxidizing agent to a silicon source gas such as SiH4 or Si(OC2H5)4 and then processed by heat or plasma energy. Its relative dielectric constant is about 4.0.
Alternatively, a fluorinated amorphous carbon film has been produced from CxFyHz as a source gas by a plasma CVD method. Its relative dielectric constant ∈ is as low as 2.0–2.4.
Another method to reduce the relative dielectric constant of insulation film has been made by using the good stability of Si—O bond. A silicon-containing organic film is produced from a source gas under low pressure (1 Torr) by the plasma CVD method. The source gas is made from P-TMOS (phenyl trimethoxysilane, formula 1), which is a compound of benzene and silicon, vaporized by a babbling method. The relative dielectric constant ∈ of this film is as low as 3.1.

A further method uses a porous structure made in the film. An insulation film is produced from an inorganic SOG material by a spin-coat method. The relative dielectric constant ∈ of the film is as low as 2.3.
However, the above noted approaches have various disadvantages as described below.
First, the fluorinated amorphous carbon film has lower thermal stability (370° C.), poor adhesion with silicon-containing materials and also lower mechanical strength. The lower thermal stability leads to damage under high temperatures such as over 400° C. Poor adhesion may cause the film to peel off easily. Further, the lower mechanical strength can jeopardize wiring materials.
Oligomers that are polymerized using P-TMOS molecules do not form a linear structure in the vapor phase, such as a siloxane structure, because the P-TMOS molecule has three O—CH3 bonds. The oligomers having no linear structure cannot form a porous structure on a Si substrate, i.e., the density of the deposited film cannot be reduced. As a result, the relative dielectric constant of the film cannot be reduced to a desired degree.
In this regard, the babbling method means a method wherein vapor of a liquid material, which is obtained by having a carrier gas such as argon gas pass through the material, is introduced into a reaction chamber with the carrier gas. This method generally requires a large amount of a carrier gas in order to cause the source gas to flow. As a result, the source gas cannot stay in the reaction chamber for a sufficient length of time to cause polymerization in a vapor phase.
Further, the SOG insulation film of the spin-coat method has a problem in that the material cannot be applied onto the silicon substrate evenly and another problem in which a cure system after the coating process is costly.
In view of the above, various techniques of forming low-k silica insulation films have been developed.
In order to reduce wiring resistance, copper wiring is widely used in combination with low-k silica insulation films. However, copper tends to migrate or diffuse into the silica insulation films. In order to prevent this problem, a hard film is required between the silica insulation film and copper wiring. Conventionally, SiC or SiN is mainly used as a hard film. However, the dielectric constant of such a hard film is relatively high, and thus when forming the hard film on the silica insulation film, the hard film increases the effective dielectric constant of the integrated layers of the hard film and the silica insulation film, even if the dielectric constant of the insulation film is low. Conventionally, as a source gas, 3MS or 4MS is used, and films having a dielectric constant of less than 4.0 have not been obtained, although a hard film having a low dielectric constant is in demand for reducing the effective dielectric constant at the low-k insulation film.
Therefore, a principal object of this invention is to provide a method for forming an improved hard film which has a low dielectric constant.
Further, properties such as fine structures, barrier effect against copper, and controlling film stress are required for hard films. Another object of this invention is to provide a method for forming a hard film that has a low dielectric constant, fine structures, and appropriate levels of film stress.
A further object of this invention is to provide a method for forming a hard film that has good mechanical strength.
A still further object of this invention is to provide a method for effectively forming a hard film without requiring complicated processes.